High speed detection of faults on multi-terminal power system transmission lines has been attempted by using digital current differential measurements. Differential techniques rely on the fact that, under normal conditions for each phase, the sum of currents entering terminals equals the charging current for that phase. In one conventional digital differential current system, the procedure is to compare individual samples or use a one-cycle window, use a conventional dual slope operate-restraint characteristic, and compensate for line charging. This system is not flexible enough to operate in both high and low bandwidth communication channels. Additionally, the sensitivity of this system is low because conventional operate-restraint characteristics are not adaptive. In another conventional digital differential current system, charges are calculated at both ends of a two terminal system by integrating respective current signals and are then compared. This system has sensitivity limitations and works only for two terminal embodiments.
Many power system monitoring, protection, and control functions could be performed more efficiently and accurately if power system digital measurements at multiple locations were synchronized. Generally such measurements are only somewhat synchronized because of difficulty in accurately synchronizing sampling clocks physically separated by large distances. Conventional uses of digital communications to synchronize sampling clocks at remote locations have accuracies limited by uncertainties in the message delivery time. In particular, digital communications can have different delays in different directions between a pair of locations which lead to an error in clock synchronization.
Variable size data windows in power system protection devices have generally been avoided because of the associated complexity, computational burden, and communications requirements. Where digital communications can have different delays in different directions between a pair of locations which lead to an error in clock synchronization.
Variable size data windows in power system protection devices have generally been avoided because of the associated complexity, computational burden, and communications requirements. Where variable size data windows have been implemented, a different set of weighting functions is used for each data window. When the data window changes size, recalculations are then required for all the samples in the data window.
Conventional power system impedance relays, including electromechanical, solid state, and digital relays, typically detect faults by calculating an effective impedance from voltage and current measurements. When the effective impedance falls within a certain range, a fault is declared. For a first zone relay, the range is typically set for less than 85-90% of the total line length impedance to allow for uncertainties in the underlying measurements of power system quantities. The actual uncertainties vary with time. Conventional impedance relays do not recognize the time varying quality of the underlying measurements, and thus sensitivity and security can be compromised.
Inherent uncertainties exist in estimating fundamental power frequency voltages and currents from digitized samples and arise from a number of sources, including, for example, power system noise, transients, sensor gain, phase and saturation error, and sampling clock error. Conventional practice is to account for these errors during system design by estimating the worst case and including enough margin to allow for the errors. The conventional procedure does not take into account the time varying nature of the errors. Other procedures to determine the sum of the squares are computationally intensive.
The standard method for providing transformer current differential protection is to develop restraint and operate signals from measured transformer currents from each winding, and to use a discrete Fourier transform (DFT) or a fast Fourier transform (FFT) to calculate various harmonics. The operate signal is usually calculated based on the principle that the sum of the ampere turns approximately equals the magnetizing current and is thus calculated as the algebraic sum of the ampere turns for each winding. The restraint signal is usually based on the fundamental frequency current or a weighted sum of the fundamental frequency current and selected harmonics to factor magnetizing inrush and overexcitation.